1. Field of the Invention
The present invention relates to a surface covering for a solar cell module in which a photovoltaic element having a photoactive semiconductor layer serving as the photoelectric converting member is protected with resin, and more particularly a solar cell module capable of showing excellent performance even under exterior conditions.
2. Relates Background Art
Among the recent worldspread consciousness of ecological issues, the deepest concern is directed to the warming of the earth by CO.sub.2 production, and the development and stable supply of clean energy are urgently desired objectives. The solar cell is one of the most promising clean energy sources because of its safety and ease of handling. Solar cells have been prepared in various forms such as (1) monocrystalline silicon solar cells; (2) polycrystalline silicon solar cells; (3) amorphous silicon solar cells; (4) copper-indium selenide solar cells; and (5) compound semiconductor solar cells. Among these types of cells, the thin film crystalline silicon solar cells, compound semiconductor solar cells, and amorphous silicon solar cells are recent targets of active development as they are relatively inexpensive and can be formed into a large area.
Even among these cells, the amorphous (non-monocrystalline) solar cells, prepared by depositing silicon on a conductive metal substrate and forming a transparent conductive layer thereon, is considered promising as a future solar cell module, as it is light in weight, excellent in impact resistance, and extremely flexible. However, different form the cells where silicon is deposited onto a glass substrate, the surface at the light entrance side has to be coated with a transparent coating material in order to protect the solar cell.
Glass and resinous material of high weather resistance, such as fluorinated resin, have been employed for such surface coating material. FIG. 3 shows an example of such solar cell module, in which there are provided a layer 304 of glass or resin of high weather resistance; a filler layer 303; a solar cell 302; a filter layer 305; and an insulating layer 301. The requirements for the coating material to be employed for surface protection of the solar cell module include high weather resistance, high moisture resistance, and high protecting ability.
In relation to the above-mentioned requirements for the coating material, the Japanese Patent Publication No. 4-7229 discloses, for a solar cell composed of a substrate and CdS/CdTe formed thereon, a protective film composed of a derivative of a resin containing perfluoroalkylene radicals and active hydrogen atoms. Luminflon (manufactured by Asahi Glass Col, Ltd.) is cited as an example of the resin containing perfluoroalkylene radicals and active hydrogen atoms. According to the above-mentioned patent specification, Lumiflon is a fluorine-containing polymer with a number-averaged molecular weight of 20,000 to 80,000, containing perfluoroalkylene radicals and pendant active hydrogen atoms, or more specifically, OH radicals, and reacts with melamine or a compound containing isocyanate radicals to form crosslinked polymer, termed a derivative in said specification. The above-mentioned patent specification also discloses, in the description of examples, the preparation of a protective film of high moisture resistance by crosslinking Lumiflon with an isocyanate or resol phenolic resin.
The coating film disclosed in the above-mentioned patent must be positioned at the outermost surface of the solar cell module. The resin, after mixing with the crosslinking agent, generally has a short pot life, which in practice has to be extend by protecting the isocyanate with a blocking agent. Thus, if a laminated structure is intended to be formed in which a surfacial film is laminated on the resin, the crosslinking reaction may not proceed properly as the blocking agent cannot be liberated and evaporated.
Also, the lamination of the surfacial film after the crosslinking of resin is difficult because the crosslinked substance lacks adhesiveness or stickiness. Moreover, an effective blocking agent is not known if melamine is employed as the crosslinking agent. For these reasons, the above-mentioned resin has to be used as the outermost surface of the solar cell module. However, such resin has a low surface hardness, generally in the order of B to H in pencil hardness, and is easily scratched by sand and dust present outdoors, and smear and dust may be deposited on such scratches to eventually block the solar light. Also such resins, if simply painted, tends to cause formation of pinholes or dust, causing introduction of moisture or oxygen to the photovoltaic element. Thus, there has not been known an organic surface coating material capable of providing a high level weather resistance and moisture resistance.
Glass coating is best for preventing moisture absorption and yellow discoloration, and has therefore been employed frequently for sealing the solar cells. However, such glass coating is poor in flexibility, impact strength, weight, and cost.
Also, the protective ability is important not only for maintaining the performance of the solar cell module itself but also for safety consideration. The protective ability of the surface coating material can be tested with a "scratch test" in which, with a testing machine shown in FIG. 7, a steel blade is moved along the surface of the solar cell, with a speed of 152.4 mm/sec. and under a load of 907 g, and the solar cell passes this test if the electrical properties thereof show no defects in the electrical test thereafter.
The Japanese Patent Laid-open Application No. 59-56776 discloses a method of producing a solar cell module, featured by preparing a composite sheet consisting of a plastic sheet (or film) having a softening point of 120.degree. C. or higher and having high optical transparency and a polyacrylic resin layer of a softening point of 120.degree. C. or lower formed thereon, and either a polyacrylic resin sheet of a softening point of 120.degree. C. or lower or a laminated sheet consisting of the resin sheet and a moisture preventing sheet, placing a solar cell element between the resin layers and applying heat and pressure to fuse or soften at least one of the acrylic resin layers, thereby sealing the solar cell element. According to the specification of the above-mentioned application, the plastic sheet (or film) can be composed of any plastic material having a softening point of 120.degree. C. or higher and being optically transparent, and can be a sheet or a film of polycarbonate, polyamide resin, hard polyvinyl chloride resin, or polyfluoroethylene resin. The recommended range of thickness is about 50 to 800 .mu.m. It is also disclosed that the surface of the sheet of film may be treated, for example, with silicon or magnesium fluoride in order to improve stress crack resistance.
However, the weather resistance is still defective even when employing a polycarbonate film as the hard film and silicone or magnesium fluoride as the surface coating material, because such surface coating material is incapable of blocking the ultraviolet light which induces deterioration and discoloration of polycarbonate. For this reason, there is no disclosure of the weather resistance of the hard film employed in the solar cell module.
For laminating these materials, there is usually employed the method of superposing the surface covering material, the photovoltaic element and the rear covering material, with thermoplastic resin used as the filler layer 301, and heating these members under vacuum.
However, the surface covering material, if composed of glass, increases the weight and the cost, though it can pass the scratch test mentioned above. On the other hand, fluorinated resin cannot pass the scratch test unless the filler is made thick enough, because the hardness of such fluorinated resin is low. Besides, if the filler is made thicker in order to pass the scratch test, the solar cell module may become combustible and unsuitable for use as a roofing material or the like. For resolving such drawbacks there should be employed a covering material of a higher hardness, instead of the fluorinated resin, but so far thee has not been known any material having weather resistance, water repellency, and which is superior to fluorinated resin in performance.